In order to perform maintenance and modifications to nuclear reactor vessel internals, it is necessary at times to perform machining on the existing internal components. One such repair involves the addition of several restraint rods, (usually four) which anchor to the lower shroud support plate area, and extend upward to connect over the top of the shroud cylinder. Although the configurations of each plant vary, therefore creating some differences in the designs of each system, there is a common need in most designs to produce holes in the shroud support plate, adjacent the jet pump diffusers for attachment of repair components. The governing nature of the reactor vessel internal components is such that essentially all work is performed in-situ, underwater (obtaining depths of up to 100 feet), keeping radiation exposure of the workers to a minimum. Foreign materials exclusion (FME) is of utmost concern due to its potentially deleterious effect on the nuclear fuel, monitoring systems, flow of coolant/moderator, etc. The working envelope is typically very restrictive, and thus, all tooling must be compact and remotely operated.
The conventional method for performing machining operations on reactor vessel internal components utilizes a metal removal process called electrical discharge machining (EDM). By creating an electrical spark across a small gap between an electrode and the workpiece, the material is essentially eroded away, producing a swarf by-product. It has been shown that any swarf left after the EDM process poses no detrimental concerns as a foreign material. As there are no real mechanical reaction forces between the workpiece and electrode, the tooling developed need not be very substantial, thereby lending itself to packaging tools within the restrictive interference envelopes.
The EDM process, though predominantly utilized, is a very slow, time consuming process. Typical best anticipated times to produce a 3 inch diameter hole in a 1 1/2 inch thick workpiece are upwards of 16 hours. Effects of ambient pressure, side arcing, electrode wear, hole breakthrough, swarf flushing and tooling failures due to long submersion times can all contribute to excessive delays during the EDM process. Moreover, inherent to the EDM process is the quick heating/cooling cycle generated with each spark. This rapid heating/cooling creates a re-cast layer on the machined surface. Depending on the material being machined, it is sometimes necessary to grind or hone the re-cast layer to remove any microfissures that may be detected.
Conventional machining inside the reactor vessel has historically not been attempted due to the generation/retention requirements of the chips produced. Thus, there is a need for improved productivity and process reliability of machining reactor vessel internals.